In this specification the term “tub” is employed as a general term meaning a wide mouth container which may be in the form of a tub, as that term may be used by some of those skilled in the art of moulded polymer containers, a tray, a pot, a jar, a cup, etc. The wide mouth of the container has an opening which has substantially the same or greater dimensions and area as compared to the body and base of the container. The “tub” may have a variety of different shapes, dimensions and aspect ratios. The invention is particularly directed to containers which have a shape and configuration, for example a cauldron shape, which means that they cannot be formed by known thermoforming processes in which a sheet or film of blown or extruded thermoplastic material is heated and then blown or impressed against the inside surface of a female mould cavity having an inner moulding surface shaped to mould the outer surface of the desired container.
In the packaging industry, the process of blow moulding is often used in the manufacture of containers, particularly bottles for carbonated beverages. This process involves the initial formation of a preform, typically by injection moulding, which preforms are subsequently blow moulded to form the containers. Such preforms are typically formed of thermoplastic material, particularly polyethylene terephthalate (PET).
For the manufacture of containers in the form of tubs, typically thermoforming is used. A sheet of thermoplastic material, typically a polyolefin, is heated and then urged, by a movable mould member and a blowing pressure, into a mould cavity. Tubs often suffer from the problem of poor mechanical properties, in particular poor impact resistance, particularly at low temperatures. This is because the thermoformed thermoplastic tends to exhibit poor molecular alignment or orientation, which may be monoaxial orientation or only a low degree of biaxial orientation. It is well known that biaxial orientation increases polymer toughness in thermoplastic packaging. However, conventional thermoforming processes tend to produce no or only low biaxial orientation, particularly in regions of the packaging which may be subjected to the greatest impact stresses during use, and so which require the greatest toughness or impact resistance.
The present invention aims at least partially to overcome these problems of known containers and corresponding container manufacturing methods. There is a need in the art for a container, and a corresponding method of manufacture, which provides cost-effective containers having dimensions to enable them to be used as tubs and which have good mechanical properties, for example impact resistance.